Green Pea Galaxies

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Green Pea Galaxies

Post by MargaritaMc » Wed Apr 03, 2013 6:49 pm

http://ns.umich.edu/new/releases/21352- ... y-universe
Green Pea galaxies could help astronomers understand early universe
April 03 2013
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ANN ARBOR—The rare Green Pea galaxies discovered by the general public in 2007 could help confirm astronomers' understanding of reionization, a pivotal stage in the evolution of the early universe, say University of Michigan researchers.

Reionization occurred a few hundred million years after the Big Bang as the first stars were turning on and forming the first galaxies. During this period, the space between the galaxies changed from an opaque, neutral fog to a transparent charged plasma, as it is today. Plasma is gas that's electrically charged.

As for how this happened, the prevailing theory holds that massive stars in the early galaxies produced an abundance of high-energy ultraviolet light that escaped into intergalactic space. There, the UV light interacted with the neutral hydrogen gas it met, blasting electrons off the hydrogen atoms and leaving behind a plasma of negatively charged electrons and positively charged hydrogen ions.

"We think this is what happened but when we looked at galaxies nearby, the high-energy radiation doesn't appear to make it out. There's been a push to find some galaxies that show signs of radiation escaping," said Anne Jaskot, a doctoral student in astronomy.

Jaskot and Sally Oey, an associate professor of astronomy in the College of Literature, Science, and the Arts, have found that the Green Peas could hold that evidence. Their findings are published in the current edition of the Astrophysical Journal.

"The Green Peas are compact, highly star-forming galaxies that are very similar to the early galaxies in the universe," Jaskot said. "Our analysis shows they may be leaking ionizing radiation."

The researchers focused on six of the most intensely star-forming Green Pea galaxies, which are between one billion and five billion light years away. They studied their emission lines as observed by the Sloan Digital Sky Survey. Emission lines show how light interacts with matter, and in this case, they helped the astronomers understand the relationship between the stars and gas in these galaxies.

The emission lines told Jaskot and Oey how much light the galaxies absorbed. Then, to determine how much light was there to start with, they ran models to estimate, for example, how old the galaxies are and how many stars they contain. The galaxies, the researchers determined, produced more radiation than the researchers detected, so they infer that some of it must have escaped.

"An analogy might be if you have a tablecloth and you spill something on it. If you see the cloth has been stained all the way to the edges, there's a good chance it also spilled onto the floor," Jaskot said. "We're looking at the gas like the tablecloth and seeing how much light it has absorbed. It has absorbed a lot of light. We're seeing that the galaxy is saturated with it and there's probably some extra that spilled off the edges."

Jaskot says the Green Peas are exciting candidates to help astronomers understand a major milestone in the development of the cosmos 13 billion years ago.

The paper is called "The Origin and Optical Depth of Ionizing Radiation in the 'Green Pea' Galaxies. The research is funded by the National Science Foundation.
Related Links:

Abstract: http://arxiv.org/abs/1301.0530
Full paper: http://arxiv.org/pdf/1301.0530v1.pdf
Anne Jaskot: http://dept.astro.lsa.umich.edu/~ajaskot
Sally Oey: http://dept.astro.lsa.umich.edu/~msoey
"In those rare moments of total quiet with a dark sky, I again feel the awe that struck me as a child. The feeling is utterly overwhelming as my mind races out across the stars. I feel peaceful and serene."
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Re: Green Pea Galaxies

Post by MargaritaMc » Wed Apr 03, 2013 7:04 pm

Because it is still near April first, I was suspicious of something that sounded so very odd! So I got some more information from Wikipedia

http://en.wikipedia.org/wiki/Pea_galaxy
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A Pea galaxy, also referred to as a Pea or Green Pea, might be a type of Luminous Blue Compact Galaxy which is undergoing very high rates of star formation.[1] Pea galaxies are so-named because of their small size and greenish appearance in the images taken by the Sloan Digital Sky Survey (SDSS).
Pea Galaxies were first discovered in 2007 by the volunteer users within the forum section of the online astronomy project Galaxy Zoo (GZ).
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"In those rare moments of total quiet with a dark sky, I again feel the awe that struck me as a child. The feeling is utterly overwhelming as my mind races out across the stars. I feel peaceful and serene."
— Dr Debra M. Elmegreen, Fellow of the AAAS

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Re: Green Pea Galaxies

Post by bystander » Wed Apr 03, 2013 7:21 pm

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alive to the gentle breeze of communication, and please stop being such a jerk.
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Does it smell like aspara-gas?

Post by neufer » Wed Apr 03, 2013 8:15 pm

MargaritaMc wrote:
http://ns.umich.edu/new/releases/21352-green-pea-galaxies-could-help-astronomers-understand-early-universe wrote:
Green Pea galaxies could help astronomers understand early universe (April 03 2013)
Anne Jaskot wrote:
"The Green Peas are compact, highly star-forming galaxies that are very similar to the early galaxies in the universe," Jaskot said. "Our analysis shows they may be leaking ionizing radiation. An analogy might be if you have a tablecloth and you spill something on it. If you see the cloth has been stained all the way to the edges, there's a good chance it also spilled onto the floor. We're looking at the gas like the tablecloth and seeing how much light it has absorbed. It has absorbed a lot of light. We're seeing that the galaxy is saturated with it and there's probably some extra that spilled off the edges."
http://www.buzzle.com/articles/green-urine-causes.html wrote:
What Does Green Pee Mean?
  • 1) Infection of the urinary tract, problems with bile and intake of certain drugs.

    2) Bright green color may be due to excess intake of vitamin B

    3) Consumption of foods like asparagus may cause urine to become true green.
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Re: Green Pea Galaxies

Post by Ann » Thu Apr 04, 2013 4:40 pm

And eating too much avocado might make "number two" turn green, or so a pair of avocado-munching acquaintances of mine have informed me... Image

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Re: Green Pea Galaxies

Post by Ann » Thu Apr 04, 2013 4:45 pm

Okay, but here's a basic fact that I haven't understood: Why are the green pea galaxies green? Are they green because we are seeing Lyman Alpha emission redshifted into the green part of the spectrum? Are they green because they emit a lot of, say, OIII emission, and this emission has been redshifted into the red part of the spectrum, which is shown as green in SDSS g-r-i images?

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Re: Green Pea Galaxies

Post by MargaritaMc » Thu Apr 04, 2013 5:05 pm

http://asterisk.apod.com/viewtopic.php?t=25679
Their green color comes from strong OIII (doubly ionised oxygen) emission lines (a common consequence of lots of new star formation) within a redshift (z) range around 0.2. A redshift of 0.2 means we see these galaxies as they were when the universe was about 2.4 billion years younger (according to Ned Wright’s cosmology calculator). Equivalent early universe galaxies are most luminous in ultraviolet at a redshift (z) between 2 and 5 – when the universe was between 10 and 12 billion years younger than today.
I found that in one of the topic links that bystander posted
"In those rare moments of total quiet with a dark sky, I again feel the awe that struck me as a child. The feeling is utterly overwhelming as my mind races out across the stars. I feel peaceful and serene."
— Dr Debra M. Elmegreen, Fellow of the AAAS

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Re: Green Pea Galaxies

Post by neufer » Thu Apr 04, 2013 5:18 pm

Ann wrote:
Okay, but here's a basic fact that I haven't understood: Why are the green pea galaxies green? Are they green because we are seeing Lyman Alpha emission redshifted into the green part of the spectrum? Are they green because they emit a lot of, say, OIII emission, and this emission has been redshifted into the red part of the spectrum, which is shown as green in SDSS g-r-i images?
Neither: http://asterisk.apod.com/viewtopic.php? ... 74#p196389
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Re: Green Pea Galaxies

Post by Ann » Thu Apr 04, 2013 6:18 pm

neufer wrote:
Ann wrote:
Okay, but here's a basic fact that I haven't understood: Why are the green pea galaxies green? Are they green because we are seeing Lyman Alpha emission redshifted into the green part of the spectrum? Are they green because they emit a lot of, say, OIII emission, and this emission has been redshifted into the red part of the spectrum, which is shown as green in SDSS g-r-i images?
Neither: http://asterisk.apod.com/viewtopic.php? ... 74#p196389
Ummmm. So you are saying that the green pea galaxies are green because of airglow, aurorae, or some sort of greenish streetlamps?

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Re: Green Pea Galaxies

Post by MargaritaMc » Thu Apr 04, 2013 6:35 pm

http://asterisk.apod.com/viewtopic.php? ... 11#p196709
http://asterisk.apod.com/viewtopic.php?t=25679
Their green color comes from strong OIII (doubly ionised oxygen) emission lines (a common consequence of lots of new star formation) within a redshift (z) range around 0.2. A redshift of 0.2 means we see these galaxies as they were when the universe was about 2.4 billion years younger (according to Ned Wright’s cosmology calculator). Equivalent early universe galaxies are most luminous in ultraviolet at a redshift (z) between 2 and 5 – when the universe was between 10 and 12 billion years younger than today.
I found that in one of the topic links that bystander posted
Ann, does this not give an answer? :?:
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Last edited by MargaritaMc on Thu Apr 04, 2013 6:40 pm, edited 1 time in total.
"In those rare moments of total quiet with a dark sky, I again feel the awe that struck me as a child. The feeling is utterly overwhelming as my mind races out across the stars. I feel peaceful and serene."
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Re: Green Pea Galaxies

Post by neufer » Thu Apr 04, 2013 6:38 pm

Ann wrote:
neufer wrote:
Ann wrote:
Okay, but here's a basic fact that I haven't understood: Why are the green pea galaxies green? Are they green because we are seeing Lyman Alpha emission redshifted into the green part of the spectrum? Are they green because they emit a lot of, say, OIII emission, and this emission has been redshifted into the red part of the spectrum, which is shown as green in SDSS g-r-i images?
Neither: http://asterisk.apod.com/viewtopic.php? ... 74#p196389
Ummmm. So you are saying that the green pea galaxies are green because of airglow, aurorae, or some sort of greenish streetlamps?
Or aspara-gas.
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Re: Green Pea Galaxies

Post by Ann » Thu Apr 04, 2013 6:41 pm

Margarita, thank you! Yes, that does answer my question.

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Re: Green Pea Galaxies

Post by Chris Peterson » Thu Apr 04, 2013 6:56 pm

Ann wrote:Okay, but here's a basic fact that I haven't understood: Why are the green pea galaxies green? Are they green because we are seeing Lyman Alpha emission redshifted into the green part of the spectrum? Are they green because they emit a lot of, say, OIII emission, and this emission has been redshifted into the red part of the spectrum, which is shown as green in SDSS g-r-i images?
The galaxies are intrinsically blue/green because of their high O[III] (500.7 nm) emission. For low redshift galaxies, we observe this as green (550 nm for z=0.1) through red (650 nm for z=0.3).
camera_filters.jpg
How the galaxies appear in images depends on the arbitrary assignment of filter channels (or derived filter channels) to red, green, and blue display channels. SDSS images consist of data taken through five photometric filters, u, g, r, i, z, with pass bands illustrated here. As is common when analyzing photometric "colors", Green Pea galaxies are identified by specific band comparisons: u-r, g-r, r-i, and r-z, not by actual color at all.

I'd say these objects look green simply because of the channel mapping adopted by Galaxy Zoo. That said, the lowest redshift galaxies in this class would actually look green to our eyes were they bright enough to stimulate color vision.
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Re: Green Pea Galaxies

Post by Ann » Fri Apr 05, 2013 2:50 am

Chris Peterson wrote:
I'd say these objects look green simply because of the channel mapping adopted by Galaxy Zoo. That said, the lowest redshift galaxies in this class would actually look green to our eyes were they bright enough to stimulate color vision.
Thanks, Chris!

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Re: Green Pea Galaxies

Post by MargaritaMc » Sat Apr 06, 2013 9:31 am

Chris wrote
SDSS images consist of data taken through five photometric filters, u, g, r, i, z,
Chris, I understand u = ultra violet, g = green, r = red, i = infrared - but not what, in the context of filters, z stands for. Can you help, please? And do you know how Sloan maps 'i' and 'z' to colours in its images?

Many thanks
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Re: Green Pea Galaxies

Post by Ann » Sat Apr 06, 2013 9:58 am

I found this graph, taken from this page.

The way I understand it, the low green curve shows the sensitivity of the ultraviolet filter of the SDSS cameras. As you can see, the sensitivity appears to be quite low. The filter apparently responds to light between 300 and 400 nm (or 3000 and 4000 Ångströms). The blue curve shows the good sensitivity of the green filter, which responds well to light between 400 and 520 nm (that seems more blue than green to me, actually). The black curve apparently shows the sensitivity of the red filter, which responds very well to light between 550 and 700 nm. The red curve shows the sensitivity of the infrared filter, which responds well to light between 700 and 810 nm. The purple-pink curve shows the sensitivity of the z filter. It is quite insensitive, and responds to wavelengths between about 810 and 1000 nm or longer.

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Re: Green Pea Galaxies

Post by Chris Peterson » Sat Apr 06, 2013 2:47 pm

Ann wrote:The way I understand it, the low green curve shows the sensitivity of the ultraviolet filter of the SDSS cameras.
Yes, however I don't think we are looking simply at the filter passbands here (as is usually the case when we post these kinds of charts) but of the filter convolved with the detector. That's why the response is much lower at the extremes, since the silicon detector stops being sensitive to UV and IR. The filters themselves probably pass close to all the light in their individual bands.
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Re: Green Pea Galaxies

Post by Chris Peterson » Sat Apr 06, 2013 2:54 pm

MargaritaMc wrote:Chris, I understand u = ultra violet, g = green, r = red, i = infrared - but not what, in the context of filters, z stands for. Can you help, please? And do you know how Sloan maps 'i' and 'z' to colours in its images?
There is a discussion of photometric filters here. There are lots of different systems (I use Johnson and Cousins filters). Not all of the letters are obvious abbreviations- some are just arbitrary.

I'm not sure that Sloan even has a standard color mapping system. The dataset itself doesn't consist of color images, but of the individual filter channels. If you download SDSS data, you would need to map it yourself to produce color images (and with more than three source channels, that mapping can be quite complex).
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Re: Green Pea Galaxies

Post by MargaritaMc » Sat Apr 06, 2013 4:37 pm

Chris wrote

There is a discussion of photometric filters here. There are lots of different systems (I use Johnson and Cousins filters). Not all of the letters are obvious abbreviations- some are just arbitrary.
Thank you very much, Chris. It helps to know that some of the letters referencing filters are just arbitrary.

The Wikipedia link you gave was excellent: I've read various articles there on filters but hadn't found that one.

The information that it gives for the filters used by Sloan is:

u' = 354 nm g' = 475 nm r' = 622 nm i' = 763 nm z' = 905 nm
I'm not sure that Sloan even has a standard color mapping system. The dataset itself doesn't consist of color images, but of the individual filter channels. If you download SDSS data, you would need to map it yourself to produce color images (and with more than three source channels, that mapping can be quite complex).
Mmm, I see. I'd be intrigued to find out who or what system gets this dataset into photographic format for classification by the Galaxy Zoo volunteers.

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Re: Green Pea Galaxies

Post by MargaritaMc » Fri Feb 21, 2014 10:29 pm

This is a recent arXiv paper about Green Pea Galaxies
The Origin and Optical Depth of Ionizing Photons in the Green Pea Galaxies
A.E Jaskot, M.S. Oey
Our understanding of radiative feedback and star formation in galaxies at high redshift is hindered by the rarity of similar systems at low redshift. However, the recently identified Green Pea (GP) galaxies are similar to high-redshift galaxies in their morphologies and star formation rates and are vital tools for probing the generation and transmission of ionizing photons. ...

http://arxiv.org/abs/1402.4429
"In those rare moments of total quiet with a dark sky, I again feel the awe that struck me as a child. The feeling is utterly overwhelming as my mind races out across the stars. I feel peaceful and serene."
— Dr Debra M. Elmegreen, Fellow of the AAAS

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